Conventional displaying is based on flat panels which cannot be bent freely. A trend of displaying is to display a large quantity of information on a flexible body, i.e., flexible displaying. Flexible display screens and flexible display modules can be mechanically bent at any step of encapsulating, manufacturing, storing, utilizing, operating, carrying or transporting substrates and between adjacent processes. Key technologies for achieving the flexible displaying mainly include: display technology, substrate technology, array technology and encapsulation technology. Currently, there exist three kinds of substrates for the flexible displaying, i.e., ultra-thin glass substrate, metal foil substrate and plastic substrate.
The ultra-thin glass substrate may provide well resistance to water and oxygen and have well transparency. However, it is sensitive to cracks and has poor impact resistance and poor flexural endurance, so it is difficult to perform a roll-to-roll process and develop a flexible ultra-thin glass substrate. The metal foil substrate may provide well resistance to water, oxygen and high temperature, together with low cost and well ductility, so it is easy to perform the roll-to-roll process. However, a surface of the foil is rough (Ra=0.6 μm), and a planarization layer is required to be coated onto the surface even when it has been polished. As a result, a device thickness will increase. The plastic substrate is more flexible, lighter and more impact-resistant, so it is suitable for the manufacture of a light and thin device, and thereby becomes a trend of the flexible display technology.
Usually, the plastic substrate is not resistant to high temperature, and it may be deformed remarkably at the high temperature. Hence, manufacturing processes for a flexible display device must be accomplished at a very low temperature. In order to achieve a high-temperature manufacturing process, it is required to select a heat-resistant plastic substrate and coat it onto a glass substrate.
A flexible display device includes semiconductor elements (e.g., thin film transistors (TFTs)) which need to be manufactured at a high temperature. During the high-temperature manufacturing process, the plastic substrate may be subjected to thermal weight loss and may release small-molecule solvents and impurities, resulting in adverse influences on the characteristics of the TFTs and thereby the image quality. A solution in the related art is to form an inorganic thin film on the plastic substrate as a barrier layer, so as to prevent the semiconductor elements from being adversely affected by the organic base plate during the high-temperature manufacturing process. However, crystal lattices of the plastic substrate and the inorganic thin film do not match (that is, material stresses do not match). Since the plastic substrate is an organic layer with a relatively large thermal expansion coefficient, it may be deformed remarkably during the high-temperature manufacturing process, and after the plastic substrate is cooled to a normal temperature, the inorganic thin film may be subjected to the thermal stress. As a result, microcracks may easily occur and the inorganic thin film may easily fall off from the plastic substrate.